Among the processes known for preparing 2-(4-isobutylphenyl)propionic acid or esters thereof is that of Shimizu et al. (U.S. Pat. No. 4,694,100, issued September, 1987), who teach the reaction of p-isobutyl-styrene with carbon monoxide and water or alcohol in the presence of a palladium catalyst and a mineral acid, e.g., HCl. This patent also teaches the alternative reaction of p-isobutylstyrene with carbon monoxide and hydrogen in the presence of a metal complex carbonyl catalyst to produce 2-(4-isobutylphenyl)propionaldehyde, which is then oxidized to produce the desired product. The preparation of the starting material for this reaction is disclosed as the reaction of isobutylbenzene with acetaldehyde in the presence of sulfuric acid, producing 1,1-bis(4-isobutylphenyl)ethane, which is then catalytically cracked to produce p-isobutylstyrene and isobutylbenzene.
Another process for preparing ibuprofen is that of European Patent Application 284,310 (Hoechst Celanese, published September, 1988), which teaches that ibuprofen can be prepared by carbonylating 1-(4-isobutylphenyl)ethanol with carbon monoxide in an acidic aqueous medium and in the presence of a palladium compound/phosphine complex and dissociated hydrogen and halide ions, which are preferably derived from a hydrogen halide. This process has the disadvantage of starting with 1-(4-isobutylphenyl)ethanol, a compound which is not economical to make by known processes.
Gardano et al. (U.S. Pat. No. 4,536,595, issued August, 1985) teach the preparation of alkaline salts of certain alphaarylpropionic acids by reaction with carbon monoxide, at substantially ambient temperature and pressure conditions, of the corresponding arylethyl secondary halide in an anhydrous alcoholic solvent in the presence of alkaline hydroxides and, as catalyst, a salt of cobalt hydrocarbonyl.
Alper et al. in J. Chem. Soc. Chem. Comm., 1983, 1270-1271, discloses that alkenes can react with carbon monoxide, water, hydrochloric acid and a mixture of palladium and copper to produce the hydrocarboxylated product, branched chain carboxylic acid. Oxygen is necessary to succeed in the reaction. Subsequently, Alper et al. have disclosed similar catalyst systems, but employing a chiral ligand as being successful in asymmetric hydrocarboxylation reactions. See Alper et al., PCT Application, WO 91 03,452 and J. Am. Chem. Soc., 112, 2803-2804 (1990).
Japanese Kokiku Patent No. SHO (1981)-35659 discloses an anhydrous method of producing a 2-(4-isobutylphenyl) propion acid ester by treating 2-(4-isobutyl-phenyl) ethanol with carbon monoxide in a solution of an alkanol and a catalyst such as palladium/bis(triphenylphosphine)dichloro complex. The solution may also contain up to ten percent (10%) of a mineral acid, such as hydrogen chloride.